A device that converts computer output into printed images. Following is an overview of the various technologies

Device used to apply inked images of alphanumeric or other symbolic characters to paper, or to duplicate an illustration ; graphic design , or photographic image on paper. A printer may be manually operated, mechanically operated, or computer driven. There are many types of printers that vary in terms of the way the image is created and the type of paper and ink used. Some printers create a text image one character at a time; others can reproduce one full page at a time of both text and illustrations. Speed, quality, and cost also vary greatly. The best printer for a job depends upon the type of image to be created, the level of quality desired, and the speed required. See also gravure; impact printer ; ink-jet printer laser printer ; lithography ; press .

computer output device that reproduces data on paper or another medium.

Impact - These printers have a mechanism that touches the paper in order to create an image.

Impact printers use a mechanical hammering device to produce each character. A formed character printer forces metal or plastic characters against an inked ribbon to produce a sharp image on paper; the characters may be on a moving bar, a rapidly rotating chain, a rotatable ball, or wheel spokes.

Nonimpact printers use thermal and electrostatic, rather than mechanical, techniques. Ink-jet printers, including bubble-jet printers, squirt heated ink through a matrix of holes to form characters or images. Laser printers form an image of the output on a selenium-coated drum using laser light that is turned on and off by data from the computer and then transfer the output from the drum using photocopying techniques. Thermal-wax-transfer printers and dye-sublimation printers use heat to transfer color pigment from a ribbon to a special paper to produce photographic-quality color images. Nonimpact printers are quieter than impact printers and produce higher quality output, especially of graphics, but at a greater cost per page. These printers do not touch the paper when creating an image. Inkjet printers are part of this group, which include

The term laser printer , is a bit more mysterious -- how can a laser beam , a highly focused beam of light , write letters and draw pictures on paper?

In this article, we'll unravel the mystery behind the laser printer, tracing a page's path from the characters on your computer screen to printed letters on paper. As it turns out, the laser printing process is based on some very basic scientific principles applied in an exceptionally innovative way.

energy that makes clothes in the dryer stick togThe primary principle at work in a laser printer is static electricity , the same ether or a lightning bolt travel from a thundercloud to the ground. Static electricity is simply an electrical charge built up on an insulated object , such as a balloon or your body. Since oppositely charged atoms are attracted to each other, objects with opposite static electricity fields cling togetherlaser printer uses this phenomenon as a sort of &quot;temporary glue.&quot; The core component of this system is the photoreceptor , typically a revolving drum or cylinder. This drum assembly is made out of highly photoconductive material that is discharged by light photons .

wire , a wire with an electrical current running through it. (Some printers use a charged roller instead of a corona wire, but the principle is the same.) As the drum revolves, the printer shines a tiny laser beam across the surface to discharge certain points. In this way, the laser &quot;draws&quot; the letters and images to be printed as a pattern of electrical charges -- an electrostatic image . The system can also work with the charges reversed -- that is, a positive electrostatic image on a negative background.

After the pattern is set, the printer coats the drum with positively charged toner -- a fine, black powder. Since it has a positive charge, the toner clings to the negative discharged areas of the drum, but not to the positively charged &quot;background.&quot; This is something like writing on a soda can with glue and then rolling it over some flour: The flour only sticks to the glue-coated part of the can, so you end up with a message written in powder.

With the powder pattern affixed, the drum rolls over a sheet of paper, which is moving along a belt below. Before the paper rolls under the drum, it is given a negative charge by the transfer corona wire (charged roller). This charge is stronger than the negative charge of the electrostatic image, so the paper can pull the toner powder away. Since it is moving at the same speed as the drum, the paper picks up the image pattern exactly. To keep the paper from clinging to the drum, it is discharged by the detac corona wire immediately after picking up the toner.

Before a laser printer can do anything else, it needs to receive the page data and figure out how it's going to put everything on the paper. This is the job of the printer controller .

The printer controller is the laser printer's main onboard computer. It talks to the host computer (for example, your PC ) through a communications port, such as a parallel port or USB port. At the start of the printing job, the laser printer establishes with the host computer how they will exchange data. The controller may have to start and stop the host computer periodically to process the information it has received.

A typical laser printer has a few different types of communications ports.

the printer passes the paper through the fuser , a pair of heated rollers. As the paper passes through these rollers, the loose toner powder melts, fusing with the fibers in the paper. The fuser rolls the paper to the output tray, and you have your finished page. The fuser also heats up the paper itself, of course, which is why pages are always hot when they come out of a laser printer or photocopier .

So what keeps the paper from burning up? Mainly, speed -- the paper passes thr

For the printer controller and the host computer to communicate, they need to speak the same page description language . In earlier printers, the computer sent a special sort of text file and a simple code giving the printer some basic formatting information. Since these early printers had only a few fonts, this was a very straightforward process.

These days, you might have hundreds of different fonts to choose from, and you wouldn't think twice about printing a complex graphic. To handle all of this diverse information, the printer needs to speak a more advanced language.

The primary printer languages these days are Hewlett Packard's Printer Command Language (PCL) and Adobe's Postscript . Both of these languages describe the page in vector form -- that is, as mathematical values of geometric shapes, rather than as a series of dots (a bitmap image). The printer itself takes the vector images and converts them into a bitmap page. With this system, the printer can receive elaborate, complex pages, featuring any sort of font or image. Also, since the printer creates the bitmap image itself, it can use its maximum printer resolution .

Some printers use a graphical device interface (GDI) format instead of a standard PCL. In this system, the host computer creates the dot array itself, so the controller doesn't have to process anything -- it just sends the dot instructions on to the laser.

But in most laser printers, the controller must organize all of the data it receives from the host computer. This includes all of the commands that tell the printer what to do -- what paper to use, how to format the page, how to handle the font, etc. For the controller to work with this data, it has to get it in the right order.

Once the data is structured, the controller begins putting the page together. It sets the text margins, arranges the words and places any graphics. When the page is arranged, the raster image processor (RIP) takes the page data, either as a whole or piece by piece, and breaks it down into an array of tiny dots. As we'll see in the next section, the printer needs the page in this form so the laser can write it out on the photoreceptor drum.

In most laser printers, the controller saves all print-job data in its own memory. This lets the controller put different printing jobs into a queue so it can work through them one at a time. It also saves time when printing multiple copies of a document, since the host computer only has to send the data once

The laser receives the page data -- the tiny dots that make up the text and images -- one horizontal line at a time. As the beam moves across the drum, the laser emits a pulse of light for every dot to be printed, and no pulse for every dot of empty space. The laser doesn't actually move the beam itself. It bounces the beam off a movable mirror instead. As the mirror moves, it shines the beam through a series of lenses . This system compensates for the image distortion caused by the varying distance between the mirror and points along the drum

The laser assembly moves in only one plane, horizontally. After each horizontal scan, the printer moves the photoreceptor drum up a notch so the laser assembly can draw the next line. A small print-engine computer synchronizes all of this perfectly, even at dizzying speeds.

Some laser printers use a strip of light emitting diodes ( LEDs ) to write the page image, instead of a single laser. Each dot position has its own dedicated light, which means the printer has one set print resolution. These systems cost less to manufacture than true laser assemblies, but they produce inferior results. Typically, you'll only find them in less expensive printers.

Laser printers work the same basic way as photocopiers , with a few significant differences. The most obvious difference is the source of the image: A photocopier scans an image by reflecting a bright light off of it, while a laser printer receives the image in digital form.

Another major difference is how the electrostatic image is created. When a photocopier bounces light off a piece of paper, the light reflects back onto the photoreceptor from the white areas but is absorbed by the dark areas. In this process, the &quot;background&quot; is discharged, while the electrostatic image retains a positive charge. This method is called &quot;write-white.&quot;

In most laser printers, the process is reversed: The laser discharges the lines of the electrostatic image and leaves the background positively charged. In a printer, this &quot;write-black&quot; system is easier to implement than a &quot;write-white&quot; system, and it generally produces better results.

One of the most distinctive things about a laser printer (or photocopier ) is the toner. It's such a strange concept for the paper to grab the &quot;ink&quot; rather than the printer applying it. And it's even stranger that the &quot;ink&quot; isn't really ink at all.

So what is toner? The short answer is: It's an electrically-charged powder with two main ingredients: pigment and plastic .

The role of the pigment is fairly obvious -- it provides the coloring (black, in a monochrome printer) that fills in the text and images. This pigment is blended into plastic particles, so the toner will melt when it passes through the heat of the fuser. This quality gives toner a number of advantages over liquid ink. Chiefly, it firmly binds to the fibers in almost any type of paper, which means the text won't smudge or bleed easily.

So how does the printer apply this toner to the electrostatic image on the drum? The powder is stored in the toner hopper , a small container built into a removable casing. The printer gathers the toner from the hopper with the developer unit . The &quot;developer&quot; is actually a collection of small, negatively charged magnetic beads. These beads are attached to a rotating metal roller, which moves them through the toner in the toner hopper.

Because they are negatively charged, the developer beads collect the positive toner particles as they pass through. The roller then brushes the beads past the drum assembly. The electrostatic image has a stronger negative charge than the developer beads, so the drum pulls the toner particles away.

Initially, most commercial laser printers were limited to monochrome printing (black writing on white paper). But now, there are lots of color laser printers on the market.

Essentially, color printers work the same way as monochrome printers, except they go through the entire printing process four times -- one pass each for cyan (blue), magenta (red), yellow and black. By combining these four colors of toner in varying proportions, you can generate the full spectrum of color.

Thermal bubble - Used by manufacturers such as Canon and Hewlett Packard , this method is commonly referred to as bubble jet . In a thermal inkjet printer, tiny resistors create heat, and this heat vaporizes ink to create a bubble. As the bubble expands, some of the ink is pushed out of a nozzle onto the paper. When the bubble &quot;pops&quot; (collapses), a vacuum is created. This pulls more ink into the print head from the cartridge. A typical bubble jet print head has 300 or 600 tiny nozzles, and all of them can fire a droplet simultaneously

Piezoelectric

i n this technology uses piezo crystals . A crystal is located at the back of the ink reservoir of each nozzle. The crystal receives a tiny electric charge that causes it to vibrate. When the crystal vibrates inward, it forces a tiny amount of ink out of the nozzle. When it vibrates out, it pulls some more ink into the reservoir to replace the ink sprayed